<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<h1>0</h1>
<script>
  let count = 0;
  let h = document.querySelector("h1");
  //开启定时器
  setInterval(f,1000);
  function f() {
    count++;
    h.innerText = count;
  }
  //匿名方法开启定时器
 let timer = setInterval(function () {
    console.log(count);
    if (count==5){
      //停止定时器
      clearInterval(timer);
    }
  },1000);

  //开启只执行一次的定时器
  setTimeout(function () {
      alert("时间到!");
  },3000);



</script>
</body>
</html>